A study finds that people who are significantly obese are at higher risk of premature death, but not those who are merely overweight. And there may even be benefits to a few extra pounds.

A study found that people who are significantly obese are at higher risk… (Rick Elkins / Getty Images )

It's a common medical refrain: Carrying extra pounds raises the risk of ills such as heart disease and diabetes and therefore the risk of a premature death.

But does that heightened risk of early death apply across the board to those who are merely overweight?

A new analysis of nearly 3 million people suggests maybe not.

The finding, published online Tuesday in the Journal of the American Medical Assn., pooled data from 97 studies encompassing adult men and women in the United States, Canada, Europe, Australia, China, Taiwan, Japan, Brazil, India and Mexico.

A total of 270,000 people died of any cause during the studies. When the scientists crunched the numbers, they found, as expected, that people who were significantly obese — with a body mass index, or BMI, of 35 or more — had shorter life spans on average than those who were of normal weight, defined as having a BMI of 18.5 to 24.9.

But the scientists also found that people classified as overweight, with a BMI of 25 to 29.9, died at slightly lower rates — not higher — than those of so-called normal weight. And they found that those who were mildly obese, with a BMI of 30 to 34.9, died in no greater numbers than did their normal-weight peers.

Study lead author Katherine M. Flegal, an epidemiologist with the Centers for Disease Control and Prevention, said she and her colleagues could not say what lay behind the apparent survival edge for overweight people. But she noted that it had been observed before in other studies.

Flegal added that smoking — which raises the risk of early death but also tends to keep people thinner — doesn't appear to be the explanation, since that factor was carefully controlled for in the analysis.

The paper didn't make any recommendations for doctors or members of the public, Flegal added. "Our goal is really to summarize existing information and not conclude what people should do, other than follow good health practices, no matter what their weight," she said.

There are a range of possible reasons why people who are overweight might fare better in studies than those who are of normal weight, said obesity researcher Dr. Steven B. Heymsfield, executive director of the Pennington Biomedical Research Center in Baton Rouge, La.

Only some of those explanations suggest that carrying a few extra pounds truly makes someone healthier.

For example, some studies suggest that people who are overweight or mildly obese are treated more aggressively by their doctors for blood pressure or cholesterol problems than those who are of normal weight, said Heymsfield, who was not involved in the study but co-wrote a commentary that accompanied it.

Preferential treatment could more than compensate for a higher rate of health problems among those who are overweight or mildly obese.

Health professionals also know that the BMI — a number that is calculated using a person's height and weight — is not a perfect indicator of how much extra fat someone carries because people who are extra muscular may score as overweight when they're not, for example, Heymsfield added.

But there also could be real reasons why carrying extra pounds could confer a survival advantage. Fatter people are not as prone to osteoporosis and have more padding to protect the bones should a patient take a tumble, lowering the risk of a life-endangering hip fracture.

And carrying extra fat provides energy reserves in cases of a severe illness. Doctors know, for example, that plumper patients with heart failure or kidney failure do better than their leaner peers, and there could be other situations in which the same thing is true.

"I think we should be open-minded and ask, 'OK, what could be helpful about fat?'" Heymsfield said.

The survival edges reported in the analysis were not large. Those who were overweight were on average 6% less likely to die during the studies than those whose BMI scored in the normal range.

Those with mild obesity — also known as grade 1 obesity — were at no increased risk of death compared with people with a normal BMI.

But those who were fatter than that (with a BMI of 35 or above) had a 29% higher risk of death during the studies.